Hand dexterity in chimpanzees is a 'selective advantage' to evaluate fruit ripeness

To select figs on the trees, chimpanzees show great dexterity with their hands, squeezing the fruits not unlike humans when they shop, scientists say.

Their research, published in the journal Interface Focus, examined apes in their natural habitat with the aim of better understanding the foraging advantages of having opposable fingers and hands that manage to grasp objects with precision, a skill also known as "advanced manual prehension".

The scientists found that chimpanzees have a distinctive advantage to choose the best food, thanks to their hands' dexterity. Past studies have linked humans' hands development with early tool use, but these new findings call attention to another selective force on the origin of advanced dexterity during primate evolution.

In Kibale National Park

The team, led by scientists from Dartmouth University, observed the behaviours of chimpanzees, black-and-white colobus monkeys, red colobus monkeys and red-tailed monkeys in Kibale National Park (Uganda). The animals looked for figs in the trees and seemed to use different mechanisms to make out which fruits were ripe.

Because figs can stay green throughout different stages of their development, the monkeys had trouble picking them out based solely on their assessment of the fruit's colour. To determine if the green figs of the tree Ficus sansibarica were edible, chimpanzees, unlike other apes, made a series of sensory assessments.

On top of evaluating the colour, they also smelled the fig, bit it, and touched it using their opposable fingers. All of this helped them determine the fruit's elasticity and softness.

The scientists then collected samples of the fruits that had been rejected, half eaten by the monkeys, or not selected at all. They determined if each fruit was ripe or not. Based on this data, they evaluated the predictive power of sensory information for chimpanzees to estimate the ripeness of figs.

According to the study's authors, squeezing figs was about four times faster than detaching and then biting the fruit. This technique appears to be a lot more efficient than any other, giving the chimpanzees a substantial foraging advantage over birds and other monkeys, which rely simply on visual and oral information.

This offers a new insight regarding the evolution of hands and opposable fingers within the primate family, including in humans. "This result explains why chimpanzees evaluate green figs by palpation and dental incision, actions that could explain the adaptive origins of advanced manual prehension", the authors conclude.